Braking System

ABSTRACT

Disclosed is a redundant braking system for a vehicle. A controller may monitor a status of a hydraulic braking system, and during vehicle travel, automatically switch to a modulated electric braking system which is separate from the hydraulic braking system when the hydraulic braking system fails. The modulated electric braking system may utilize a brake acting on a drive shaft of the vehicle, and an operator-actuated switch may set a mode of the modulated electric braking system to a non-modulated parking brake mode.

The present invention is related to a redundant braking system and amethod of operating the redundant braking system. The redundant brakingsystem provides electrically operated brakes for various types ofvehicles.

On-road vehicles and off-road vehicles, for example, may range in sizefrom less than 10,000 lbs gross vehicle weight (GVW) and smaller, andmay range in size of up to 25,000 lbs GVW and greater. Brakes forvehicles (parking brakes, for example) in these size ranges may operateby acting upon an output shaft of a vehicle's transmission or transfercase, and may use a spring mechanism to engage a brake (for example, aspring mechanism of a spring brake). To release the brake, compressedair may be used to overcome a holding force of a spring of the springmechanism. Parking braking systems may be essentially binary. That is,if a parking brake is binary, the brake is either fully engaged or fullyreleased.

Such a braking system may also have a fail-safe feature which operatessuch that if air pressure is lost for any reason, the brake willimmediately lock on. Once a brake is locked on, the brake may not bereleasable unless air pressure is restored or a holding force of thespring mechanism is overcome by intervention, such as, for example,mechanical intervention.

Although a braking system may have a spring brake which is configured tobe essentially fail-safe (locking on when air pressure is lost), such aspring brake may have some limitations as far as providing modulatedbraking control. Modulated braking, also referred to as progressivebraking, transfers an amount of friction or braking action to the wheelsor output shaft of a vehicle, which is proportional to pressure appliedto a brake pedal, for example. In other words, during modulated braking,as more braking force is applied to the brake pedal, more force isapplied to the wheels or output shaft of the vehicle. In a case whereinmodulated braking control is not provided, the vehicle will simply cometo an abrupt stop when the brake goes into fail-safe mode. This could bea significant issue for military vehicles or other applications thatrequire operational flexibility. Operational flexibility may includecapacity for continued control of the vehicle in an event of a failureof a system of the military vehicle. Operational flexibility may beachieved through the use of redundant or back-up systems, includingback-up braking systems.

During a military operation, for example, if an air system of a vehiclefails or is lost due to enemy fire, or due to some other circumstance,the spring brakes may automatically lock on. In other words, the brakesare configured to “fail safe”. Locking on of the spring brakes couldhave an undesirable effect of immobilizing the vehicle. In the militaryoperation, for example, during any combat situation, an immobilizedvehicle could become an easy target, and the safety of the vehicle, thevehicle's occupants and the vehicle cargo could be put in jeopardy.

One of the primary means of controlling the speed of a vehicle duringordinary everyday use is by using the vehicle's service brakes. Thedesignation “service brakes” is generally used to describe modulatedbrakes of a vehicle which apply “as needed” force to the wheels of avehicle during normal use of the vehicle, e.g., when the vehicle is “inservice”.

For military vehicles, especially tactical military vehicles, aredundant braking system may have a failure mechanism which could resultin the vehicle remaining in a fully mobile and fully operable conditionin the event of a failure. A tactical military vehicle may be a vehicleused within, or in direct support of, tactical forces. Tactical forcesmay be forces involved in support of combat operations, or to forcesengaged in actual combat.

An electric redundant braking system may allow for modulated control ofthe vehicle (for example, modulated braking) in the event of anemergency such as a loss of service brakes or when the use of compressedair is not possible or not desirable. For example, a use of compressedair may not be desirable during operation of a hybrid-electric vehiclewhen the hybrid-electric vehicle is in a mode in which it uses anelectric motor only. A hybrid-electric vehicle is a vehicle which uses amixture of power and propulsion technologies such as internal combustionengines, electric motors, diesel or gasoline and batteries. When in“electric mode”, hybrid-electric vehicles typically may not have a needto generate compressed air.

An electric redundant braking system may be configured to operate byacting on a vehicle's drive shaft. In the electric redundant brakingsystem, a braking apparatus can be used as a back-up service brake inthe event that the main service brakes fail or are not used for someother reason, or the redundant braking apparatus can be used as a simpleparking brake. The arrangement of an assembly comprising the redundantbrake at the drive shaft may be referred to as a brake apparatus orbrake mechanism.

The electric redundant braking system may be implemented on a vehiclehaving an internal combustion (IC)-electric hybrid drive capable ofbeing periodically operated in an electric-only mode (with the IC engineturned off). When the vehicle is operated with the IC engine turned off,use of air-assisted brakes can be problematic since the IC engine (whichmay be used to generate compressed air to operate the air assistedbrakes) is no longer being used. When the IC engine is not used, anelectric motor may be used instead.

An electrically operated brake may use the same stored electrical powerthat is used for vehicle propulsion during a mode in which an electricmotor is used to run the vehicle instead of the IC engine. When thestored electrical power is used for vehicle propulsion, compressed airmay not be needed to operate the brakes. Also, since an electric brakeis usually operated only on an as needed basis, the current draw fromthe batteries which power the electric brake is quite low.

In one embodiment, a vehicle uses a parallel diesel hybrid-electricdrive. In this exemplary embodiment, when the vehicle is powered by theinternal combustion engine, the service brakes are air over hydraulicbrakes. In an operation of the air over hydraulic brakes, compressed airis used to activate a master cylinder of the hydraulic system, which inturn facilitates application of a brake to each wheel of the vehiclethrough hydraulic pressure. In an air over hydraulic mode, a servicebrake pedal modulates an amount of compressed air acting on the mastercylinder, thus providing an operator with a means by which to determinean amount of brake force required to control the vehicle by the familiarmethod of adjusting the amount of pressure applied to the brake pedal(for example, an amount of foot pressure applied to the brake pedal).

In an implementation such as the one described above, the redundantelectric brake may provide a braking system having a failure mechanismwhich is different from and fully independent of a failure mechanism ofthe air over hydraulic system described above. In the air over hydraulicsystem, for example, a loss of air pressure or a loss of hydraulic fluidmay cause the air over hydraulic system to fail. The electric system, onthe other hand, may fail if electrical power is lost to the brakemechanism.

The electric redundant braking system may be implemented to operate in aback-up mode which uses modulated signals from the vehicle's brake pedalto apply the electric brake apparatus during vehicle operation tosupplement or replace a non-electrical service brake. The back-upbraking mode can be activated either manually by the operator usingbuttons, switches or a keypad, for example, or automatically activatedwhen a service brake failure is detected. Alternatively, the back-upbraking mode can be activated remotely by an operator. An operator maybe a person residing in a command center, a driver of the vehicle or apassenger in the vehicle. An operator may be another device such as asmart device activating the back-up braking mode based on combinationallogic.

The back-up mode may be activated either automatically or manually whena mode of operation of the vehicle is switched to a silent mode ofoperation. The designation “silent mode” may be used to refer to a modein which little or no noise is produced or detectible from the vehicle.For example, when operating in silent mode, the vehicle may generatelittle or no engine noise or little or no brake noise. Silent mode mayalso be used to describe a mode in which a radar signature or thermalsignature of a vehicle is very small, that is, the vehicle is notdetectible by radar or heat imaging means.

The electric redundant braking system may be implemented to operate in aparking brake mode, wherein the electric brake is applied and lockedinto place through a ratcheting mechanism that maintains brake forceeven after electric power to the brake mechanism is cut. The parkingbrake may be operated using buttons or switches located on a dashboardof a vehicle, or located in any location which is accessible to anoperator of the vehicle. The operator may be a driver, a passenger, or acontroller remote from the vehicle, for example.

The electric redundant braking system may make use of sensors which areinstalled on the compressed air system and the master cylinder. Thesensors may measure air pressure, hydraulic fluid level and/or hydraulicfluid pressure. If a predetermined amount of change occurs in either theair pressure, the hydraulic fluid level, the hydraulic fluid pressure,or any combination of the above, the system may automatically activatethe electric redundant braking system using the electric brake mechanism(a disc brake, for example) on the transmission output shaft.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a view of a parallel diesel hybrid-electric vehicle inwhich the present invention may be implemented.

FIG. 2 shows the electric brake components of the present invention ingreater detail, including the parking brake ratcheting mechanism.

FIG. 3 shows a flow chart for a method of operating a braking system ofthe present invention.

DETAILED DESCRIPTION

Embodiments of the redundant braking system will be described withreference to the drawings.

For the purpose of illustration, the electric redundant braking systemis described within the context of an implementation in a paralleldiesel hybrid-electric vehicle 1 using a parallel diesel hybrid-electricpropulsion system with 4-wheel drive as shown in FIG. 1. The electricredundant braking system may be implemented in a variety of vehicletypes having differing propulsion and drive systems, and in some cases,in vehicles having propulsion and drive systems which are the same, forexample, in a case which propulsion and/or drive systems are bothelectrical.

Referring to FIG. 1, the parallel diesel hybrid-electric vehicle 1includes a diesel engine 40, which may be used as a primary propulsionsystem for the parallel diesel hybrid-electric vehicle 1, and anelectric motor 41 which may be used as a back-up propulsion system forthe parallel diesel hybrid-electric vehicle 1. The parallel dieselhybrid-electric vehicle 1 includes an air over hydraulic braking system32 for the front and rear brakes. The air over hydraulic braking system32 includes service brake pedal 4 for applying a braking force frominside the parallel diesel hybrid-electric vehicle 1. A master cylinder13 is filled with a required level of hydraulic fluid (brake fluid). Afluid level detector 14 is installed at the master cylinder 13 to detecta fluid level in the master cylinder 13. A fluid pressure sensor may beinstalled at the master cylinder 13 as well, in order to detect apressure of the fluid in the master cylinder 13. A pressure vessel 11 isconnected to the master cylinder 13. A pressure sensor 12 is installedat the pressure vessel 11 to detect and transmit a pressure thereof. Thefluid level detector 14 and pressure sensor 12 transmit fluid levelinformation and pressure information to a system status monitor 10.Installed at wheels 44 are discs 24 and calipers 50.

The parallel diesel hybrid-electric vehicle 1 includes a drive shaft 42connected to differentials 17, 18 and 19, which transfer torque to thewheels 44. The differentials 17, 18 and 19 may comprise lockingdifferentials or slip differentials. A transfer case 31 transfers torquefrom either the diesel engine 40 or the electric motor 41 to the driveshaft 42, including output shaft 15. Installed on the output shaft 15 isa disc 16 on shaft, servomotor 2 and a clamp or caliper 26. The disc 16on shaft, servomotor 2 and clamp or caliper 26 comprise a brakeapparatus or brake mechanism 30. The constituent parts of brakeapparatus 30 are not limited to the disc 16 on shaft, servomotor 2 andclamp or caliper 26, but may be comprised of a variety of constituentparts acting on the drive shaft 42 of the parallel dieselhybrid-electric vehicle 1 including output shaft 15. For example,another device could be used instead of the servomotor 2 to provide aforce to the disc 16 on shaft to stop or reduce a rotation of the outputshaft 15. For example, the braking mechanism 30 may use electro-magneticmeans, or an arrangement such as a toroid arrangement to stop, or reduceor modulate a rotation of the output shaft 15.

A controller 6 is connected to a brake pedal force and pressuretransducer 5, which is connected to brake pedal 4. A block and bleedsolenoid valve 3 is connected between the master cylinder 13 and thebrake pedal 4. The block and bleed solenoid valve 3 receives an outputsignal from the controller 6. The controller 6 outputs operating mode orstatus information to a mode display 7 which displays a variety ofinformation, including mode information such as normal mode or silentmode, and status information such as service brakes in use, park brakeon, and back-up brakes in use. A hand operated mode selector switch 9used to switch between modes. The parallel hybrid-electric vehicle 1 mayhave several hand operated mode selector switches 9, installed in alocation convenient to the operator. The hand operated mode selectorswitches 9 may be used to switch between a normal operating mode and aback-up mode. The hand operated mode selector switches 9 may be used toswitch to a silent mode while continuing to operate in the normaloperating mode or the back-up mode. The hand operated mode selectorswitches 9 may be used to switch a mode of operation between the airover hydraulic brakes to the electric brake apparatus. A battery orbatteries 8 may be used to power the entire vehicle including theelectric braking system. The energy source for the electric redundantbraking system is not limited to batteries only. Rather, the energysource for the electric redundant braking system may be any sourcecapable of providing electric power.

The brake pedal 4, brake pedal force and pressure transducer 5,controller 6, mode display 7, mode selector switches 9, batteries 8,system status monitor 10, servomotor 2 and braking mechanism 30 comprisean electric braking system, and the electric braking system issubstantially or completely separate from the air over hydraulic brakingsystem.

FIG. 2 provides a more detailed view of the parking brake mechanism 30.Referring to FIG. 2, servomotor 2 is connected to clamps 26 which applypressure to parking brake pads 25. These in turn apply pressure to discat wheels 24. The ratchet paw 23 engages with ratchet gear 21 whichmaintains its locked position even when the servomotor 20 isde-energized, and thus the parallel hybrid-electric vehicle 1 may not bemovable unless the ratchet paw 21 is disengaged. The solenoid 22 engagesand disengages the ratchet paw 21. The mode selector switch 9 may beused to send a signal to the solenoid 22 to disengage the ratchet paw.The signal from the mode selector switch 9 releases the parking brake,and then the parallel hybrid-electric vehicle 1 can be operated usingmodulated braking control. The parking brake may also be manuallyreleased. For example, ratchet paw 21 may be connected to a wire, andthe wire may be connected to a push-pull mechanism or lever locatedinside the parallel hybrid-electric vehicle 1. A push or pulling actionon the mechanism or lever releases the parking brake.

For added parking brake effectiveness, it is preferred that the threedifferentials 17, 18 and 19 be of a locking differential type or be ofthe limited-slip differential type. The electric redundant brakingsystem can also be implemented on a 4-wheel drive system without thecenter differential 18 or in a 2-wheel drive vehicle with a single reardifferential 17. For added effectiveness, all of the differentials usedmay be locking type differentials, or be limited-slip typedifferentials.

An operation of the redundant braking system will now be described.

In one embodiment, when the parallel diesel hybrid-electric vehicle 1 isoperating in a normal operating mode and being propelled under the powerof diesel engine 40, the brakes in use are the air over hydraulic brakes32. During operation, an operator may use mode selector switch 9 toswitch to a silent mode. In silent mode, propulsion power for theparallel diesel hybrid-electric vehicle 1 may be provided by theelectric motor 41. For operation in silent mode, the controller 6 may beconfigured to automatically switch to a back-up mode in which the onlybraking system in use is the electric braking system, whereby thebraking mechanism 30 applies a modulated braking force to the outputshaft 15. When the controller 6 switches to the electric braking system,it also outputs a signal to the block and bleed solenoid valve 3 toclose, and thereby block a master cylinder 13 side of the block andbleed solenoid valve 3. Simultaneously, the block and bleed solenoidvalve 3 vents or bleeds the brake pedal 4 side to atmosphere. Thisprevents simultaneous operation of both the air over hydraulic brakes 32and the electric braking system. Alternatively, the controller 6 may beprogrammed to leave block and bleed solenoid valve 3 open, therebyallowing simultaneous operation of both the air over hydraulic brakesand the electric brakes. In such a case, the electric brakes would serveas auxiliary modulated brakes.

When the parallel hybrid-electric vehicle 1 is switched to silent modeand the electric braking system is also in use, the mode display 7indicates that the parallel diesel hybrid-electric vehicle 1 isoperating in silent mode and that the electric brake system is in use.During modulated braking control, the brake pedal 4 and the brake pedalforce and pressure transducer 5 employ necessary feedback systems togive a vehicle operator the same “feel” at the brake pedal that waspresent during the normal mode in which the air over hydraulic brakeswere being used.

In another embodiment, when the parallel diesel hybrid-electric vehicle1 is operating in a normal operating mode and being propelled under thepower of diesel engine 40, the brakes in use are the air over hydraulicbrakes 32. During operation of the parallel hybrid-electric vehicle 1,the pressure sensor 12 and/or the level sensor 14 detects an abnormalcondition such as low pressure or a low level. A low pressure and/or lowlevel signal are transmitted to the system status monitor 10, whichtransmits low pressure or low level signal information to the controller6. In response, the controller 6 switches a mode of operation from theair over hydraulic braking system to the electric braking system.

When the controller 6 switches a mode of operation from the air overhydraulic braking system, the mode display 7 indicates that the electricbrake system is in use. The mode display 7 may also provide an audiblesignal, such as beeping sound, to inform that a mode of operation haschanged. During modulated braking control, the brake pedal 4 and thebrake pedal force and pressure transducer 5 employ necessary feedbacksystems to give a vehicle operator the same “feel” at the brake pedalthat was present during the normal mode in which the air over hydraulicbrakes were used.

In another embodiment, when the parallel diesel hybrid-electric vehicle1 is operating in a normal operating mode and being propelled under thepower of diesel engine 40, the brakes in use are the air over hydraulicbrakes 32. During operation, an operator of the parallel dieselhybrid-electric vehicle 1 may use mode selector switch 9 or a remotemeans to manually switch the vehicle operation to silent mode. Thecontroller detects that the vehicle operation has now been switched tosilent mode, and then informs the operator via the mode display 7.

However, in this instance, the system does not automatically switch fromthe air over hydraulic braking system to the back-up electric brakingsystem. Rather, the operator manually switches to the back-up electricbraking system.

When the operator manually switches to the back-up electric brakingsystem, the mode display 7 indicates that the parallel dieselhybrid-electric vehicle 1 is operating in silent mode and that theelectric brake system is in use. The mode display 7 may also provide anaudible signal, such as beeping sound, to inform that a mode ofoperation has changed. During modulated braking control, the brake pedal4 and the brake pedal force and pressure transducer 5 employs necessaryfeedback systems to give a vehicle operator the same “feel” at the brakepedal that was present during the normal mode in which the air overhydraulic brakes were used.

When the operator of the parallel diesel hybrid-electric vehicle 1presses on the brake pedal assembly 4, the force applied to the brakepedal assembly and position information of the brake pedal assembly areconverted to an electrical signal, which the controller 6 provides tothe servomotor 2, which then applies a clamping force to the disc 16 onshaft in proportion to the force applied by the operator to the brakepedal assembly 4 and in proportion to the resultant amount of travel ofthe brake pedal assembly 4.

For a parking brake mode of operation, the operator of the paralleldiesel hybrid-electric vehicle 1 may manually activate the parking brakeusing the mode selector switch 9, which sends a signal to the servomotor2 via controller 6 to apply the maximum available clamping force to thedisc 16 on shaft. When the parking brake circuit is energized, the modedisplay 7 provides an indication that the parking brake is in use. Themode display 7 may also provide an audible signal, such as beepingsound, to inform that a mode of operation has changed.

A method of operating a braking system of an embodiment of the presentinvention is shown in FIG. 3. In the flowchart of FIG. 3, a vehicle isnormally operated using modulated hydraulic braking. During operation,the hydraulic braking system is monitored in order to detect a failure.If a failure is not detected, the vehicle continues to operate using thehydraulic brakes. If a failure is detected, the braking system isswitched to the electric braking system. The vehicle then operates usingmodulated electric braking control via the braking mechanism 30 actingon a drive shaft of the parallel hybrid-electric vehicle 1.

A braking system for a vehicle in one embodiment may comprise ahydraulic braking system configured to apply brake force through a firstbrake apparatus, a modulated electric braking system completely separatefrom the hydraulic braking system and configured to apply brake forcethrough a second brake apparatus, and a control system for monitoring astatus of said hydraulic braking system and automatically switching tothe modulated electric braking system from the hydraulic braking systemduring vehicle travel when the hydraulic braking system fails.

In another embodiment, a braking system for a vehicle under control ofan operator may comprise a hydraulic braking system, a modulatedelectric braking system completely separate from the hydraulic brakingsystem, an operator actuated switch for switching the vehicle to asilent mode of operation, and a controller for switching from thehydraulic braking system to the modulated electric braking systemresponsive to actuation of the switch.

A method of operating a braking system of a vehicle in one or moreembodiments may comprise sensing a failure of a hydraulic braking systemduring vehicle travel, and automatically actuating an electric brakingsystem responsive to said sensing to provide modulated electric brakingduring continued vehicle travel, the electric braking system beingcompletely separate from the hydraulic braking system, wherein thehydraulic braking system applies modulated force through a first brakeapparatus, the electric braking system applies modulated force through asecond brake apparatus, and the first brake apparatus is completelyindependent of the second brake apparatus.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes and modifications may bemade without departing from the spirit and scope of the invention.

1. A braking system for a truck comprising: a hydraulic braking systemon the truck, the hydraulic braking system configured to apply brakeforce to the truck through a first brake apparatus; a modulated electricbraking system on the truck, the modulated electric braking system beingseparate from the hydraulic braking system and configured to apply brakeforce to the truck through a second brake apparatus completely separatefrom the first brake apparatus; and a control system for monitoring astatus of said hydraulic braking system and automatically switching tothe modulated electric braking system from the hydraulic braking systemduring truck travel in response to a predetermined amount of change inat least one of air pressure, hydraulic fluid level, and hydraulic fluidpressure; wherein the first brake apparatus is not actuable by themodulated electric braking system and the second brake apparatus is notactuable by the hydraulic braking system.
 2. The braking system of claim1, wherein the modulated electric braking system is configured to actdirectly on a drive shaft of the truck.
 3. The braking system of claim2, wherein the second brake apparatus is located on an input side of atorque transfer unit.
 4. The braking system of claim 1, furthercomprising an operator-actuated switch for setting a mode of themodulated electric braking system to a non-modulated parking brake mode.5. The braking system of claim 4, wherein in the parking brake mode amechanism of said first brake apparatus maintains a braking force to adrive shaft of the truck when electric power is removed.
 6. The brakingsystem of claim 1, wherein the hydraulic braking system is an air overhydraulic system and wherein the automatically switching to themodulated electric braking system from the hydraulic braking systemduring truck travel is in response to a predetermined amount of changein air pressure in the air over hydraulic system.
 7. The braking systemof claim 1, wherein the automatically switching to the modulatedelectric braking system from the hydraulic braking system during trucktravel is in response to a predetermined amount of change in hydraulicfluid pressure in the hydraulic braking system.
 8. The braking system ofclaim 1, wherein the control system is adapted to prevent simultaneousoperation of the hydraulic braking system and the electric brakingsystem.
 9. The braking system of claim 1, further comprising an operatoractuable switch for selecting a silent mode of vehicle operation,wherein the control system switches to the modulated electric brakingsystem from the hydraulic braking system in response to actuation of theswitch.
 10. The braking system of claim 1, wherein the predeterminedamount of change is less than an amount of change associated withcomplete failure.
 11. The system of claim 1, wherein the automaticallyswitching comprises disabling the hydraulic braking system.
 12. Abraking system for a land vehicle under control of an operatorcomprising: a hydraulic braking system configured to apply brake forcethrough a first brake apparatus; an electric braking system separatefrom the hydraulic braking system and configured to apply modulatedbrake force through a second brake apparatus completely separate fromthe first brake apparatus; an operator actuated switch for switching thevehicle to a silent mode of operation; and a controller for switchingfrom the hydraulic braking system to the modulated electric brakingsystem responsive to actuation of the switch during vehicle travel,wherein the controller is adapted to allow simultaneous operation of thehydraulic braking system and the electric braking system during vehicletravel.
 13. The system of claim 12, wherein the simultaneous operationoccurs when the vehicle is in a mode other than the silent mode.
 14. Thesystem of clam 12, wherein the land vehicle is a truck.
 15. A system forbraking a truck, the system comprising: a foot brake pedal configured toreceive a continuously variable force applied to the foot brake pedal;converting means operatively coupled to the foot brake pedal forconverting the continuously variable force to an electronic signal of amagnitude varied in response to a position of the foot brake pedal; abrake force apparatus located on an input side of a torque transfer unitand configured to apply brake force directly to a drive shaft of thetruck, the brake force apparatus including a disc brake attacheddirectly to the drive shaft; and a control unit configured to controlthe brake force apparatus to apply continuously variable modulated brakeforce to the drive shaft of the truck via the disc brake in an amountresponsive to a travel distance of the brake pedal.
 16. The system ofclaim 15, further comprising an operator actuated switch for switchingthe truck to a silent mode of operation.
 17. The system of claim 15,further comprising a finger-actuated button accessible to a driver ofthe vehicle, the button configured to set a mode of the system to anon-modulated parking brake mode in which the control unit causes thebrake force apparatus to apply and maintain a constant parking brakeforce to the drive shaft via the disk brake, the parking brake forcebeing maintained at a constant level even if the truck is powered off.18. The system of claim 15, wherein the brake force is proportional tothe travel distance of the brake pedal.
 19. The system of claim 15,further comprising a hydraulic brake system configured to apply brakingforce through a separate brake force apparatus at wheels of the truck.20. The system of claim 15, further comprising a finger-actuated switchadapted to enter the vehicle into a silent mode of operation, thecontrol unit configured to control the brake force apparatus in responseto actuation of the finger-actuated switch.